question about ecommerce

Hey
Im building a website (wordpress blog) with only one product for sale, i dont know what plugin to use
been searching all over the web.

when a buyer comes to my web he may choose this service

Your current level: --- Your desired level:----- Price:------- Buy(button)
accordingly to his current level and desired level the price will be calculated

I'm ready to pay if its some complicated set up to use.

thanks in advance

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